The field of the invention is the filtering of digital signals. More specifically, the invention relates to a new method for filtering digital signals transformed into sub-bands.
In signal processing, it is common practice to transform a source signal from the temporal domain to the frequency domain. A transformation such as this has many advantages. It makes it possible to concentrate the energy of the signal on a reduced number of coefficients, and to carry out various processing operations as a function of the application, taking into account psychovisual or psychoacoustic criteria, maximum flow-rate and other criteria.
These techniques of transformation therefore find application in numerous fields. They can be used to reduce the bit-rate of a digital sound signal, for broadcasting (for example of the DAB or digital audio broadcasting type), or for storage on optical disks or on magnetic media. They also can be applied to the reduction of the bit rate of still or moving pictures. For example, they can be applied to 4.2.2 television, high-definition television, earth digital television, visiophony, telecopies, the storage of images, and other applications such as the conversion of image formats. More generally, such techniques are also used to make changes in the representation of the signal, multiplexing and demultiplexing operations such as in radar signal processing.
The invention can be applied to all of the above-mentioned fields, and to many other fields, and to many other fields as well.
Among the known transformation techniques, there are notably the block transform techniques, such as the DCT (discrete cosine transform), FFT (fast Fourier transform), DST (discrete sine transform), KLT (Karhunen and Lowe Transform) and other types; transformation techniques using the application of a bank of filters such as QMF (quadrature mirror filters), CQF (conjugate quadrature filters), PQMF (pseudo quadrature mirror filters), and many other techniques using banks of filters implementing FPR (finite pulse response) type as well as IPR (infinite pulse response) type filters. Many other means of transformation are also known and are compatible with the method of the present invention.
From a general point of view, all these transformations have the common feature of transposing the source signal towards a representation that is frequential in varying degrees while, at the same time, preserving a spatial aspect with the transformation being done locally in the signal.
Subsequently, all these techniques will be grouped together, for purposes of simplification, under generic term of "transformation into sub-bands". These transformations into sub-bands shall therefore be seen as the application of a bank of analysis filters followed by a sub-sampling for the forward transformation. For the reverse transformation, they will be seen as an over-sampling followed by a bank of synthesis filters and an addition of all the outputs of these synthesis filters.
For the forward transformation, the sub-sampled output of each filter shall be designated by the term "sub-band". This term naturally covers sub-bands delivered by a bank of filters but also covers any type of signal delivered by any transformation which may be considered to be sub-bands, and covers signals delivered by a blockwise transformation, also called transformed coefficients.
The usefulness of splitting up a signal into sub-bands is based upon the fact that these sub-bands are sampled at a frequency below the sampling frequency of the source signal. It is clear that it is easier to carry out processing operations on sampled sub-bands which are at a lower frequency than on the original signal. It is also easier because usually, only certain sub-bands undergo a particular processing operation.
In a standard way, as shall be seen in greater detail hereinafter with reference to FIGS. 1 to 8, the processing of the sub-bands according to the known prior art methods, consists simply of the multiplication of certain of the sub-bands by a given weighting coefficient. This enables the reduction, and even the cancellation, of certain of the sub-bands.
However, these techniques have one major drawback. During the reconstruction of the signal, namely after the attenuation of certain sub-bands, "pollution" terms appear (see for example FIG. 8, terms 81 to 84) due to the phenomenon of aliasing. Techniques described below have been found to attenuate the extent of the aliasing terms. However, these techniques imply a larger number of aliasing terms with lower values. Furthermore, the filtering operations performed by means of these known techniques are often highly selective in frequencies. This prompts ripple phenomena around contours, notably in the case of image processing.
The invention provides a method for filtering a signal transformed into sub-bands that can be used to obtain a reconstituted signal that is undisturbed or little disturbed by aliasing terms. The filtering method permits the use of a very large number of filtering profiles associated with one or more sub-bands, particularly profiles of the following types: low-pass, high-pass, pass-band, rejection, and multi-band of the type accentuating or attenuating certain bands.
The invention also provides a method based on a bank of filters that is simple to implement, easy to install, particularly on an integrated circuit and greatly limits the number of multiplications needed for implementation.
The invention also provides a method of performing an adaptive filtering operation and of varying the adaptive filtering operation as a function of a maximum bit rate or of an information element related to the analysis of the signal to be filtered.
The invention also provides a method of smoothing or attenuating filtering profiles that introduce no contour ripple phenomena (in image processing) which are generally prompted by highly selective filters such as PQMF filters, used to split signals into sub-bands.
The invention can be used in various applications, including sound or image (still or moving image) processing, multiplexing, multidimensional signal processing, or other types of applications. The invention makes it possible carry out conversions of image formats, notably for interleaved images, without introducing flickering into the image. The invention may be implemented in integrated circuits.